Orthopedics

Original Research 

INCLINATION OF THE ACETABULAR CUP IN ERECT POSTURE RADIOGRAPHS

Veijo Hoikka; Mauno Ylikoski; Antti Eskola; Seppo Santavirta

Abstract

ABSTRACT

Inclination of the acetabular component was measured in a series of 100 patients who underwent total hip replacement. The radiographs were taken with the patients in erect posture. Two different reference lines were used in the measurements: the ischial reference line, and the true horizontal line perpendicular to a plumb-line used as a vertical reference line. The values measured using these two reference lines correlated well with each other, though there were important differences caused by pelvic tilt. Often an inclination that was within the normal range when measured, using the ischial tuberosity line as a reference, shifted excessively to horizontal or vertical when control measurements were performed using the true horizontal line as the reference. We conclude that inclination of the acetabular cup should be measured in erect posture radiographs, because pelvic tilt increases the degree of inclination on the side of pelvic adduction.

Abstract

ABSTRACT

Inclination of the acetabular component was measured in a series of 100 patients who underwent total hip replacement. The radiographs were taken with the patients in erect posture. Two different reference lines were used in the measurements: the ischial reference line, and the true horizontal line perpendicular to a plumb-line used as a vertical reference line. The values measured using these two reference lines correlated well with each other, though there were important differences caused by pelvic tilt. Often an inclination that was within the normal range when measured, using the ischial tuberosity line as a reference, shifted excessively to horizontal or vertical when control measurements were performed using the true horizontal line as the reference. We conclude that inclination of the acetabular cup should be measured in erect posture radiographs, because pelvic tilt increases the degree of inclination on the side of pelvic adduction.

Inclination of the acetabular cup (the cup angle in the frontal plane) has been determined using various reference lines. Horizontal or vertical reference lines have been assessed using the ischial tuberosity line,'·2 midline of the body,s and transverse plane of the body assessed as the line through the caudal limits of the sacroiliac joints.4 In some studies, no reference lines were determined.57 In all of the above-named studies, inclination of the acetabular cup was measured from radiographs taken with the patient in the supine position. Turula et al8 measured the inclination of the acetabular cup in erect posture radiographs, and found out that determination of the cup inclination using the ischial tuberosity line or true horizontal line as references gave different values. However, they did not present statistical analysis of this phenomenon. Salvati et al7 considered an inclination angle of 35° to 55° as neutral.

In the present study, we have analyzed the acetabular inclination in erect posture radiographs using two different reference lines: the ischial tuberosity line and the true horizontal line.

PATIENTS AND METHODS

One hundred patients who underwent a cementless total hip replacement were included in the study. Seventy-one of the patients were women and 29 were men. The mean age of the patients was 56 years (range: 22 to 83). Sixtynine operations were primary arthroplasties and 3 1 were revisions.

The criterion for selecting patients for the study was ability to stand erect with straight knees and without pelvic rotation. Radiographs were taken in each case in the erect posture 3 to 6 months postoperatively.9·1" No compensation for leg length inequality was used when taking the radiographs. Inclination of the acetabulum was measured using two reference lines: a plumb-line was used as the vertical reference line, and ischial tuberosity line was used as the horizontal reference line of the pelvis (Fig 1 ). The true horizontal line was assessed as a perpendicular line of the plumb-line. Pelvic tilt was measured as the angle between the ischial tuberosity line and the true horizontal line. The student's f-test was used in the statistical analysis.

Fig 1: Inclination of the acetabular component measured using the ischial tuberosity line (ITL) and the true horizontal line (HL) as reference lines. The true horizontal line is perpendicular to plumbline (PL). Due to pelvic tilt caused by functional leg length inequality, the inclination using HL as the reference line is greater than the inclination recorded using the ITL as the reference (58° and 53°, respectively).

Fig 1: Inclination of the acetabular component measured using the ischial tuberosity line (ITL) and the true horizontal line (HL) as reference lines. The true horizontal line is perpendicular to plumbline (PL). Due to pelvic tilt caused by functional leg length inequality, the inclination using HL as the reference line is greater than the inclination recorded using the ITL as the reference (58° and 53°, respectively).

Fig 2: Inclination (I, degrees) of the acetabular cup using the ischial tuberosity line as a reference line (N = number of hips).

Fig 2: Inclination (I, degrees) of the acetabular cup using the ischial tuberosity line as a reference line (N = number of hips).

Fig 3: Inclination of the acetabular cup (I, degrees) using the true horizontal line as the reference (N = number of hips).

Fig 3: Inclination of the acetabular cup (I, degrees) using the true horizontal line as the reference (N = number of hips).

RESULTS

The mean ( + SD) inclination of the acetabular component was as follows: ischial tuberosity line as the reference 46. \" ( + 7.8); true horizontal line as the reference 46.5° ( +6.7) (Figs 2-3). These values correlated well in the statistical comparison (r=.850, /><.001). In 12 cases where the inclination was between 35* and 55° when the ischial tuberosity line was used as the reference, this inclination measured less than 35° or more than 55° when the true horizontal line was used as the reference. In two cases where the inclination was less than 35° or more than 55° when using the ischial tuberosity line as the reference, the inclination measured between 35° and 55° when the true horizontal line was used as the reference. The mean (+SD) pelvic tilt was 3.0° ( + 2.8; range: 0° to 12°) (Table).

DISCUSSION

Normal acetabular cup inclination has not been definitely determined. Usually a cup angle of 45° in the frontal plane has been considered ideal. Lewfrtnek et aP recommended 40° + 10° to prevent dislocation of the femoral component from the acetabular cup. Salvati et al7 considered normal inclination to be 45° + 10°. Coventry et al6 suggested that excessive inclination may lead to increased frequency of prosthetic dislocations, but others1·3 have found no correlation between acetabular cup inclination and dislocations. In our series there were no dislocations.

In the present series with erect posture radiographs, there was no statistical difference in the inclination between when the ischial tuberosity line and the true horizontal line were used as references. However, some important differences were found. Twelve cases in which the acetabular component was in neutral position, as defined by Salvati et al,7 when measured using the ischial tuberosity line as the reference, shifted in excess to vertical or horizontal when the true horizontal line was used as the reference. At the same time, two cases that were outside the normal range of inclination, as assessed with the ischial tuberosity line as the reference, shifted to normal when measured with the true horizontal line as the reference. These differences were due to pelvic tilt caused by leg length inequality (Fig 1 ).

Wroblenski11 demonstrated that wear of the acetabulum cup is often situated on the lateral part of the cup. Visuri12 pointed out that lengthening of the operated leg was the most important single factor predicting aseptic loosening of the prosthesis. These adverse events might be due to the adducted position of the hip on the side of the prosthesis caused by leg length inequality, because leg length inequality causes tilting of the pelvis in the frontal plane.10 On the side of the functionally longer leg, the hip is in adduction which causes increased load through the hip joint on the side of adduction; the maximal load is directed toward the outer edge of the acetabu]urn9.13-16

We conclude that inclination of the acetabular cup should be measured in erect posture radiographs, because leg length inequality causes pelvic tilt, which increases the degree of inclination on the side of adduction of the pelvis.

Table

TablePELVIC TILT MEASURED AS THE ANGLE BETWEEN THE ISCHIAL TUBEROSITY LINE AND THE TRUE HORIZONTAL LINE

Table

PELVIC TILT MEASURED AS THE ANGLE BETWEEN THE ISCHIAL TUBEROSITY LINE AND THE TRUE HORIZONTAL LINE

References

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2. Gore DR. Murray MP, Gardner GM. Sepie SB. Roentgenographs measurements after Müller total hip replacement. Correlations among roentgenographic measurements and hip strength and mobility. J Bone Joint Surg. 1977;59A:948-953.

3. Lewinneck GE, Lewis JL, Tarr R. Compere CL. Zimmerman JR. Dislocation after total hip replacement arthroplasties. J Bone Joint Surg. 1 978; 60A:2 1 7-220.

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9. Gofton JP. Studies in osteoarthritis of the hip: Part IV, Biomechanics and clinical considerations. Can Med Assoc J. 1971; 104:1007-1014.

10. Hoikka V. Paavilainen T. Lindholm TS. Turuka KB. Ylikoski M. Measurement and restoration of equality in length of the lower limbs in total hip replacement. Skeletal Radiol. 1987; 16:442-446.

11. Wroblenski BM. Direction and rate of socket wear in Chamley low-friction arthroplasty. J Bone Joint Surg. 1985: 67B:757-761.

12. Visuri T. Long-term results and survivorship of the McKee-Farrar total hip prosthesis. Arch Orthop Trauma Surg. 1987; 106:368-374.

13. Fessier H. Load distribution in a model of a hip joint. J Bone Joint Surg. 1957; 39B:145-I53.

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15. Maquet CJ. Biomechanics of the Hip Heidelberg: Springer- Verlag; 1985:39-41.

Table

PELVIC TILT MEASURED AS THE ANGLE BETWEEN THE ISCHIAL TUBEROSITY LINE AND THE TRUE HORIZONTAL LINE

10.3928/0147-7447-19931201-06

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